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These visas were for petitions filed during the April 2016 H-1B filing period, which were approved for individuals to start work on October 1, 2016 (the start of 2017 fiscal year).
As per NFAP, the decline somewhat shows industry trends, particularly in digital services like cloud computing and artificial intelligence, because of which less manpower is required to serve US clients.
Other than this, the fall in numbers could be because of IT services companies choosing to hire more local talent than international.
As the lottery system goes, the more applications a company files, the higher are its chances of obtaining more visas. It simply means that the reason behind India-based companies getting fewer visas is the less number of applications filed by them.
(Image source JawBreaker NYC)